Judging by most accounts that I consider credible and the photos that I have seen, the original Alsatians were sable colored, and the other color varieties (ex; Black, Black and Tan, Red and Tan) were derived from the sables through selective breeding.

Based on what I've seen while getting acquainted with the GSD people in my area, most of the local breeders here are breeding European working lines into their own lines. The desired back is longer and straighter, with just enough croup (angulation) to give the dog maximum leverage when leaping and driving. I believe that it's actually a minority of breeders (big-timers who control most of the money) that influence the standards.

Temperaments vary in every litter. Selecting the right pup from a breed that fits your lifestyle is vital. Often, buyers select pups with high drive from working parents because they want that over-the-top dog, only to complain about living with the beast later. They pass over other pups in the same litter that have better temperaments. Eveyone wants that bold little pup that isn't afraid of anything and chases everything, but they somehow think that the grown dog will be laid back and aloof.

 Quote: John Vanek

What was the original rationale for judges favoring the sloped back?

FAD!
There is no functional purpose for any excess in sloped back or excess angulation.

I read somewhere that the increased angulation and sloping of the back helps to elongate the stride, which is what the show people favored. The GSD's long, striding gait was considered unique, and the show people want to make it more pronounced.

 Quote: Duane Hull

I read somewhere that the increased angulation and sloping of the back helps to elongate the stride, which is what the show people favored. The GSD's long, striding gait was considered unique, and the show people want to make it more pronounced.

I could agree with that if the back was as supple as a Cheetah's but it's not. The cheetah's back is quite long and in no way sloped. Most of the ground covering in the show line GSD is from the excess angulation and reach.
The downside to that over angulation is the dog can't walk at a normal pace without it's legs, mostly rear, getting all wobbly and cow hocked. The rear legs are way behind the dog and can't correctly supporting any gait other then the "flying trot"The ideal trot in a dog is when the rear foot lands in the track of the front foot. The show line GSD over reaches it's front with the back by a bunch and that often creates a dog that crabs in order to get it's front out of the way of the rear.

Bob says it well, and it makes me want to cry.

 Quote: Connie Sutherland

Bob says it well, and it makes me want to cry.

DITTO!!!